1. Field of the Invention
The present invention relates to a head for the circular extrusion of concentric layers of thermoplastic materials (whether different or not), particularly for the extrusion of tubular multi-layer films.
2. Description of the Prior Art
It is known that the use of films or sheets consisting of multiple layers and prepared by the tubular coextrusion technique is largely increasing. It is also known that the heart of the coextrusion technique is the head, many types of which are known, each having certain advantages and drawbacks. The simplest type of head consists conventionally of an inner cylindrical male body which is surrounded by a plurality of outer rings kept together be means of radial screws on a support body.
The inner male body and outer rings are concentric and the outer surface of the male body is slightly separated from the ring inner surface, whereby an annular passage is thus brought about, which imparts initial diameters and thicknesses to the tubular layers of the different materials under extrusion. The concentric pieces forming the annular passages, however, raise several problems of design, working, assembly and seal. Since the number of feeding channels is equal to the number of thermoplastic materials to be extruded, each piece requires a specific design. This raises serious constructive problems in that delicate work must be performed on machine tools in order to achieve the specific tolerances required for the different plates and channels. Further difficulties occur during the head assembly and disassembly, for example, in the cleaning of the various channels. Also, when it is desired to increase the number of tube forming layers, it is necessary to increase the number of concentric channels and thus the head outer diameter, which creates problems of cumber and operation since the heating elements for the head are located on its exterior. Thus, a good temperature control can be obtained on the outer portion of the head but not on the inner zones of the head. Further, control of the different layers thicknesses depends on the tolerances of the various channels located at the different circumferences within the annular body of the head, making it necessary to control thickness with a single adjustment with doubtful results and further constructive complications.
Several solutions have been proposed to eliminate or at least reduce these drawbacks. However, no definitive success has to date been reached. Applicant has already developed certain commercial solutions (although with some limitations).
One prior art die, covered by U.S. Pat. No. 4,280,801 to Wheeler, Jr. et al., issued July 28, 1981, includes a plurality of "bottle-shaped" cavities and distribution cavities which extend around the central axis of the die, each corresponding to one of a plurality of thermoplastic layers to be extruded onto a wire passing through the center of the die. The cavities and distribution passages diminish in length uniformly from both sides of the inlet passageway along their circumference to a point farthest from the inlet passageway, i.e., 180.degree. from the inlet passageway, at which point the bottle shaped cavity disappears entirely.
As shown in FIG. 4A, of the present application the bottle-shaped cavity of the Wheeler, Jr. et al. die has four zones (1-4) in which the molten thermoplastic material has a very low velocity, creating high speed gradients in central zones (5-6) where the velocity of the thermoplastic material is relatively high. Thus, not only are zones 1-4 unuseful as "dead zones", but they create undesirable turbulence and thus unevenness in material flow.